The present invention relates to an optical disk apparatus which serves as an external memory unit or a data file for a computer to record and reproduce a large quantity of data at high speed.
Research and development are currently under way to seek wider applications and higher performance with regard to optical disks. Optical disks constitute a recording medium with a high recording density and hence with a very large storage capacity. As an example of its application, CD-ROM drive units are finding wide acceptance for use in computers to reproduce data.
The CD-ROM drive unit is an application of the music-playback compact disk (CD) for use as a read-only memory. Operated under the system for reproducing data at a constant linear velocity (CLV), the 12-cm-diameter optical disk is used as a recording medium with a large memory capacity of about 600M bytes. In addition to the read-only CD-ROM, other products emerging from the research and development efforts are the CD-WO of write once type and the CD-R which is rewritable a number of times.
Description will first be made of the conventional optical disk apparatus for reproducing data at a constant linear velocity (CLV) by taking the CD-ROM drive unit as an example. FIG. 18 is a block diagram of the conventional optical disk apparatus. In FIG. 18, data recorded in string form on an optical disk 1 is read by an optical pickup 2, while a servo controller 3 causes the pickup 2 to follow the runout and eccentricity of the optical disk 1. The data signal is detected and digitized by a reproduced signal detector 4 into a binary signal.
A maximum time span detector 22 detects the maximum time span (11 channel clocks long with a CD-ROM) of the bit pattern from the reproduced signal, and then compares the maximum time span with a count calculated according to an output of a first reference oscillator 21. Since the maximum time span corresponds to the linear velocity in reproduction made, if the result of comparison shows that the maximum time span is shorter than the linear velocity, the maximum time span detector issues a rotation control signal to increase the rotating velocity of the disk spindle motor 7, or if the maximum time span is longer, issues a rotation control signal to decrease the rotating velocity of the spindle motor. A spindle driver 8 drives a spindle motor 7 in response to a rotation control signal.
A synchronizing clock generator 9 for generating a synchronizing clock for the reproduced signal roughly controls the rotating velocity of the spindle motor 7 so that the reproduced signal comes within a range in which it can be pulled into synchronism. For this synchronizing clock generator 9, a circuit generally called a phase locked loop (PLL) is used. The PLL circuit includes a phase comparator 11a which compares the phase of a binary reproduced signal with the phase of an output of a voltage controlled oscillator 10 (hereinafter called VCO for short) and outputs a phase difference signal, a loop filter 12 which removes high-frequency noise from the phase difference signal and decides the response characteristics of the PLL, and VCO 10 varies the oscillation frequency of the output clock according to output voltage of the loop filter 12. Thus, the PLL generates a synchronizing clock for reproducing data. As described herein, after the motor velocity is roughly controlled, the synchronizing clock generator 9 pulls the reproduced signal into synchronism and generates a synchronizing clock for use in reproducing a recorded signal.
In the optical disk 1, a data string called a frame is recorded in every block, and at the leading end of each frame, two bit patterns with the maximum time span are recorded consecutively, which are called a frame synchronizing signal. After the synchronizing clock generator 9 has pulled the spindle velocity into the pull-in enable range, for control of the rotation of the optical disk 1, the frame synchronizing signal is controlled so that its intervals are constant to make the optical disk 1 rotate at constant linear velocity (CLV) precisely.
The operation of rotation control of the optical disk 1 by a frame synchronizing signal will be described in greater detail below. A frame detector 19 detects a frame synchronizing signal based on a synchronizing clock generated from a binary reproduced signal. Since the frame synchronizing signal is generated with a period corresponding to the linear velocity during reproduction, a phase comparator 11b detects a phase difference between the frame synchronizing signal and the phase of an output of a second reference oscillator 20, and controls the spindle motor so that the period of the frame synchronizing signal is constant. The two portions enclosed by the long and short dash lines in FIG. 18 are respectively referred to as a synchronizing clock generator 9 and a disk rotation controller 15.
FIG. 19 is a diagram showing the relation between the disk radial position and the rotating velocity under a conventional 4-fold-accelerated constant linear velocity (CLV) control. As shown in FIG. 19, even if the reproducing position changes with the radial movement of the optical pickup 2, the linear velocity is precisely controlled so as to be always constant as the number of disk revolutions is controlled so as to continuously vary with changes of the reproducing position on the optical disk 1 by means of the synchronizing clock generator 9 and the disk rotation controller 15. The relation between the disk radial position and the number of disk revolutions when the linear velocity is to be constant is obtained by the following equation.N=(60 nV)/(2πr)   Eq. (A) 
where N: number of revolutions [rpm]
n: n-fold standard velocity
V: disk linear velocity at standard velocity [m/sec]
r: radial position on the disk [m]
In the conventional configuration mentioned above, however, in the event that the disk radial positions from 25 mm to 50 mm are accessed using FIG. 19, if the disk, which rotates at standard linear velocity of 1.3 m/sec, is driven at a velocity four times the standard velocity, the number of revolutions for the linear velocity to be constant at the disk radial positions of 25 mm and 50 mm is 1986 rpm and 993 rpm, respectively, as calculated from Eq. (A). It follows therefore that when the disk is accessed from the radial position of 25 mm to 50 mm, data cannot be reproduced correctly unless the number of revolutions is decreased from 1986 rpm to 993 rpm.
(1) Since the linear velocity must be made constant, when the reproducing position of the optical pickup varies as in a seek, a considerable delay occurs until the rotating velocity of the spindle motor varies to follow the changes of the pickup position, with the result that access time has to be excessively long.
(2) In order to quickly vary the rotation of the spindle motor, a large torque is required, so that a large amount of power is consumed, heat is generated in the spindle motor and it is difficult to reduce the size of the apparatus.
In JP-A-6-119710, a method is proposed, wherein data is demodulated according to the clock signal used when data was reproduced, so that during a seek, the rotating velocity just before the seek is maintained and when the optical pickup arrives at the target position, data starts to be reproduced and the disk rotating velocity is gradually matched to a specified linear velocity.
Nevertheless, because the rotating velocity of the spindle motor increases as the data transfer rate is increased, the above-mentioned problem has yet to be solved drastically.
The present invention has been made to solve the conventional problem and has as its object to propose an optical disk recording method and an optical disk apparatus, which reproduce with high-speed access and reduced power consumption the data recorded on the optical disk at a constant linear velocity (CLV) and, when the optical disk is used in the form of a writable disk, data is recorded on the optical disk using a combination of CAV control and CLV control with greater advantages of high-speed access and low power consumption.